1. Field of the Invention:
This invention relates to a pneumatic actuator, and more particularly to a pneumatic actuator coupled to a valve stem for producing a rotational movement of the valve stem.
2. Description of the Prior Art:
In the prior art, pneumatic actuators have been disclosed for converting the linear motion of a flexible diaphragm to a rotary movement of a valve stem. Generally, the pneumatic actuators of the prior art couple the diaphragm to the valve stem by means of a piston having one or more grooves in the periphery thereof, engaging pins fixed to the body of the piston and connected to the valve stem. Thus, according to the conventional design, a pressurization of the diaphragm produces a vertical movement thereof, which in turn produces a reciprocal movement of the piston, and thereby a rotational response in the valve stem.
However, in the operation of the conventional pneumatic actuator, considerable frictional forces are experienced during the operation of the piston assembly, which mandates a relatively high diaphragm pressurization force. Furthermore, the piston assembly itself is fairly large, and thus the conventional pneumatic actuator becomes impractical for those applications where space is at a premium. Furthermore, the pneumatic actuators of the prior art often employ additional tensioning devices for biasing the diaphragm towards one of its terminal positions. Thus, a considerable number of parts are used in the conventional pneumatic actuator assembly, and accordingly the actuator cost and reliability is proportionately adversely affected.
Examples of pneumatic actuators found in the prior art are U.S. Pat. No. 3,929,058 and U.S. Pat. No. 3,758,069.
In order to overcome the shortcomings of pneumatic actuators found in the prior art, an improved pneumatic actuator has been devised and is disclosed in copending U.S. Patent Application Ser. No. 957,072 filed Nov. 2, 1978 by Rawstron et al., incorporated by reference herewith. This improved pneumatic actuator described in more detail hereinafter, employs a hollow cap-shaped cam having opposed contoured slots. This cam is rigidly attached to a linearly propelled resilient diaphragm within the actuator. A rotatable pin in communication with the slots is coupled to a shaft penetrating the actuator housing, such that a linear motion of the cam results in a rotational force being applied by the cam slots against the rotatable pin, and therefore a corresponding rotation of the shaft.
While the fundamental concept of using the diaphragm and diaphragm cam in combination with the former preventing the latter from rotating, is acceptable where rotational stopping positions are not critical, e.g. for actuating ball valves, this concept may suffer in applications where precise angular rotation is mandatory because a degree of torsional compliance in the diaphragm is normal and may result in cam rotation.